The concept of a wireless sensor network (WSN) refers to a communications network that comprises a (potentially large) number of autonomous nodes that are capable of setting up and maintaining multi-hop wireless communication connections through an randomly deployed arrangement of nodes. Contrary to general-purpose ad-hoc wireless networks such as WLANs (Wireless Local Area Networks), a WSN does not aim at maximizing wireless medium utilization or at ensuring highest possible data rates. A major concern in WSNs is the long-term power consumption of the nodes, which means that even relatively modest data rates and relatively long latencies may be accepted if they help to minimize the mean amount of consumed electric power. The word “sensor” in WSNs comes from the fact that traditionally a major application area of WSNs was considered to be relative static measurement networks, in which a large number of sensor-equipped nodes act as source nodes generating measurement data, which is collected to a relatively small number of sink nodes. The sink nodes may also act as gateways that set up and maintain communications between the WSN and other communications networks.
Prior art of WSNs and related features are known at least from publications WO2006/067271, US 2004/0100917 A1, US 2003/0152041 A1, US 2002/0044533 A1, CA 2 311 245 A1, WO01/69279, WO01/26329, and U.S. Pat. No. 6,208,247 B1. Known protocols for wireless sensor networks include the Sensor-MAC (also known as S-MAC), the Self-organizing Medium Access Control for Sensor networks (SMACS), the Traffic Adaptive Medium Access (TRAMA) protocol, and the IEEE 802.15.4 Low Rate Wireless Personal Area Network (LR-WPAN) standard. Of these, the S-MAC has been described in the scientific publication W. Ye, J. Heidemann, and D. Estrin: “Medium access control with coordinated, adaptive sleeping for wireless sensor networks,” ACM/IEEE Trans. Networking, vol. 12, no. 3, pp. 493-506, June 2004. SMACS has been described in K. Sohrabi, J. Gao, V. Ailawadhi, and G. J. Pottie, “Protocols for self-organization of a wireless sensor network,” IEEE Personal Communications, vol. 7, no. 5, pp. 16-27, Oct. 2000. TRAMA is described in V. Rajendran, K. Obraczka, and J. J. Garcia-Luna-Aceves, “Energy-efficient, collision-free medium access control for wireless sensor networks,” in Wireless Networks, vol. 12, no. 1, February 2006, pp. 63-78. A further development to LR-WPAN is known as the ZigBee and described online on the official website of the ZigBee alliance (http://www.zigbee.org).
Aiming at ultimate savings in the energy needed to operate a node typically means that wireless communications in a WSN consist of short activity periods and long idle periods, during which most electric circuits in the node are in a sleep mode. Keeping the activity period short is easy, when timing is synchronized throughout the network and each node knows its immediately neighboring nodes. These assumptions hold reasonably well if mobility of nodes is low. Problems arise if the network should support highly mobile nodes, which may be the case for example in applications like access control, assets tracking, and interactive games.
FIG. 1 illustrates schematically the transmission of data between a source node and a destination node in a WSN utilizing a synchronous MAC (Medium Access Control) protocol. Here “destination” does not necessarily mean the final destination of data; this example only shows transmission of data between two nodes that are within each other's radio coverage. White blocks indicate transmission and reception of beacon signals. A simple hatch indicates a period when data reception is possible (the receiver is on), and a cross hatch indicates transmission of data. The beacon transmission 101 of the destination node is received at the source node in 102. According to the principles known from e.g. publication WO2006/067271, the beacon transmission contains all the information that the source node needs to know about the destination node in order to successfully transmit data thereto. After the beacon transmission 101 there occurs a data reception period 103 in the destination node. The source node utilises this to make its data transmission at 104. The same pattern is repeated at steps 111, 112, 113, and 114. In the meantime the source node may make its own beacon transmission at 105, and have a reception period 106. These are not necessary, if the source node is a so-called subnode of the destination node.
FIG. 1 also illustrates the concepts of a wakeup period Twakeup, which consists of an active period Tactive and a sleep period Tsleep. For reasons of graphical clarity the relative lengths of the time periods are not realistic in FIG. 1. Typically the length of the active period Tactive is considerably less than one second, while the sleep period Tsleep may be several seconds or even minutes.
The fact that the MAC protocol is synchronous means that the source node knows, when it may expect the beacon transmission 101 to come, so that the source node only needs to keep its receiver on for receiving beacon signals at that very moment. The source node has obtained the necessary information earlier by performing a so-called network scan. In principle the source node would not even need to receive the beacon transmission at 102 or at least not at 112, if it can deduce the appropriate moment for transmitting data to the destination node from some beacon signal it has previously received from the destination node. However, it is usually advisable to receive all beacon signals, because they can also contain up-to-date information about the reservation of slots by other nodes during the reception period 103 or 113, or other actual information. Regularly receiving beacon transmissions also helps to compensate for random errors in clock frequency between nodes.
Let us imagine that the source node moves, and eventually goes out of the radio coverage of the destination node. In that case the source node should find some other node close enough to its new location to communicate with. In other words the source node must make a network scan. Even if there is a common network beacon frequency on which all beacon transmissions are made, in the worst case the source node must keep its beacon receiver on for the duration of a whole wakeup period to receive even a single beacon transmission. The on-time of the receiver that is required for performing a network scan becomes even longer, if the node must listen to a number of frequencies in sequence. The more mobility there is among the nodes, the more frequent will be the need for scanning the network, which may dramatically increase the overall energy consumption of a WSN.
An objective of the present invention is to present a method, an arrangement and a computer program product for enhancing the support for mobility in a WSN without considerably increasing the overall energy consumption of the network. Another objective of the invention is that the enhanced support for mobility can be implemented in the framework of existing WSN protocols without requiring major changes. A yet another objective invention is to keep the requirements for hardware complicatedness at nodes reasonable despite of the enhanced support for mobility.
The objectives of the invention are achieved by including information about second-hop neighbor nodes in beacon transmissions, and by utilising previously received information about second-hop neighbors when creating new connections.
According to a first aspect of the invention, a node device for a wireless sensor network comprises:                a receiver for receiving transmissions from other nodes in said wireless sensor network,        a controller configured to selectively switch on said receiver according to a timetable known to said controller, and        a memory configured to store information about other nodes in said wireless sensor network;wherein said node device is configured to maintain synchronization with and receive beacon transmissions from another node in said wireless sensor network.        
As a characterizing feature said controller is configured to read from received beacon transmissions information about neighboring nodes with which said node device does not maintain synchronization and to store such information into said memory, and said controller is configured to utilize such stored information to selectively switch on said receiver to attempt receiving a beacon transmission from such a neighboring node as a response to an observed failure in previously maintained synchronization.
According to a second aspect of the invention, a node device for a wireless sensor network comprises:                a receiver for receiving transmissions from other nodes in said wireless sensor network,        a transmitter for making transmissions to other nodes in said wireless sensor network, and        a controller configured to selectively switch on said receiver and said transmitter according to a timetable known to said controller;wherein said node device is configured to maintain synchronization with and receive beacon transmissions from another node in said wireless sensor network.        
As a characterizing feature said controller is configured to compose a synchronization data unit that contains information about such node with which said node device maintains synchronization, and said controller is configured to make said transmitter transmit said synchronization data unit as a part of a beacon transmission.
A method according to a first aspect of the invention is characterized by the steps performed by a node device according to a first aspect of the invention.
A method according to a second aspect of the invention is characterized by the steps performed by a node device according to a second aspect of the invention.
A computer program product according to a first aspect of the invention is characterized in that it comprises executable instructions to make a computer perform the method according to the first aspect of the invention.
A computer program product according to a second aspect of the invention is characterized in that it comprises executable instructions to make a computer perform the method according to the second aspect of the invention.
Although the topology of a WSN does not place any requirements about any nodes being stationary, a significant probability still exists that the new node, with which a moving node will want to communicate after losing a link with a previous neighboring node, already was somewhere in a relatively concise neighborhood around the moving node's previous location. In its previous location, the moving node received beacon transmissions from immediately neighboring nodes and thus knew about them. The “neighbor awareness” of the moving node can be significantly extended outwards by making said immediately neighboring nodes announce, preferably in the same beacon transmissions that the nearby nodes will receive anyway, important information about those nodes that a particular moving node does not hear yet but that are only one step away in the network topology.
If a common network beacon frequency exists for making all network beacon transmissions, the most important thing to be announced about potential next-step neighboring nodes is the relative timing of their beacon transmissions in relation to those of the node that is making the announcement. Thus a moving node will immediately know, at which time it should expect a beacon transmission from the next-step neighboring node. If beacon transmissions come on different channels, also channel identification is among the signaled information that a node advantageously receives in beacon transmissions. This way the moving node may manage to avoid having to make full network scans for long periods. Significantly reducing the average occurrence of network scans will mean dramatic savings in the average power consumption of a WSN where nodes are allowed to move frequently.
The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb “to comprise” is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.